Typically, a solid oxide fuel cell (SOFC) employs a solid electrolyte of ion-conductive oxide such as stabilized zirconia. The solid electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly (MEA). The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, normally, predetermined numbers of the electrolyte electrode assemblies and the separators are stacked together to form a fuel cell stack.
As the fuel gas supplied to the fuel cell, normally, a hydrogen gas produced from hydrocarbon raw material by a reformer is used. In general, in the reformer, a reforming raw gas is obtained from a hydrocarbon raw fuel of a fossil fuel or the like, such as methane or LNG, and the reforming raw gas undergoes steam reforming to produce a reformed gas (fuel gas).
In the above steam reforming, water in correspondence with a quantity of water vapor used in the reforming reaction needs to be supplied. For this purpose, an approach where a required quantity of water is supplied from the outside has been adopted. Alternatively, a water collection approach by condensing the exhaust gas produced as a result of power generation in the fuel cell to achieve perfect circulation (water self-sustaining operation) of water needed for reforming has been drawing attention. In this regard, it is required to remove impurities from the condensed water. Therefore, water treatment equipment, e.g., an ion exchanger has been adopted.
For example, as shown in FIG. 13, a fuel cell device disclosed in Japanese Laid-Open Patent Publication No. 2008-243598 (hereinafter referred to as conventional technique 1) includes a fuel cell 1a, a reforming unit 2a for producing a fuel gas, raw fuel supply means 3a for supplying a raw fuel to the reforming unit 2a, oxygen-containing gas supply means 4a for supplying an oxygen-containing gas to the reforming unit 2a, and a water pump 5a for supplying water to the reforming unit 2a. Further, the fuel cell device includes water treatment means for treating water to be supplied to the reforming unit 2a, and a control unit 6a for controlling the reforming unit 2a to perform steam reforming during steady operation.
Under the control of the control unit 6a, based on a signal from water quality detecting means 7a provided between the water treating means and the reforming unit 2a, if it is determined that the water quality of the water to be supplied to the reforming unit 2a has been degraded, the supply of water by the water pump 5a is stopped. Further, the reforming unit 2a is controlled to perform partial oxidation reforming by using the raw fuel supplied by the raw fuel supply means 3a and the oxygen-containing gas supplied by the oxygen-containing gas supply means 4a. 
Further, as shown in FIG. 14, a fuel cell device disclosed in Japanese Laid-Open Patent Publication No. 2011-029116 (hereinafter referred to as conventional technique 2) includes a heat exchanger 1b for performing heat exchange between an exhaust gas and water and a condensed water supply pipe 3b for supplying condensed water produced by heat exchange at the heat exchanger 1b toward a reformer 2b. A valve 4b is provided in the condensed water supply pipe 3b, and an electric conductivity sensor 5b for measuring electrical conductivity of the condensed water is provided between the heat exchanger 1b and the valve 4b. 
The fuel cell device includes a control device 6b. In the case where the electric conductivity of the condensed water measured by the electric conductivity sensor 5b has a predetermined value or less, the control device 6b controls the valve 4b for supplying the condensed water toward the reformer 2b. In the case where the electric conductivity of the condensed water measured by the electric conductivity sensor 5b has a predetermined value or more, the control device 6b controls the valve 4b for discharging the condensed water.
Further, as shown in FIG. 15, a fuel cell device disclosed in Japanese Laid-Open Patent Publication No. 2011-249185 (hereinafter referred to as conventional technique 3) includes a fuel cell stack 1c for generating electrical energy by reactions of a hydrogen and an oxygen and discharging a gas containing water vapor, a condenser unit 2c for condensing the water vapor by cooling the gas to produce the condensed water, and a clean water supply unit 3c for supplying clean water.
Further, the fuel cell device includes a tank 4c for mixing the condensed water produced by the condenser unit 2c and the clean water supplied from the clean water supply unit 3c and storing the mixed water as raw material water, a filter 5c for purifying the raw material water, an electric conductivity measuring unit 6c for measuring electric conductivity of the raw material water after the raw material water has passed through the filter 5c, and a controller 7c. The controller 7c has functions of a first detection unit for detecting the end of the life of the filter 5c based on the measurement result in the electric conductivity measuring unit 6c, a supply quantity measuring unit for measuring the quantity of the clean water supplied from the clean water supply unit 3c, and a second detection unit for detecting the end of the life of the filter 5c based on the measurement result in the supply quantity measuring unit.